Process for drying materials of high moisture diffusion resistance



United States Patent PROCESS FOR DRYING MATERIALS OF HIGH MOISTURE DIFFUSION RESISTANCE Marcello Thomas Giachino, Union City, N. J., assignor to Commercial Solvents Corporation, Terre Haute, Ind., a corporation of Maryland No Drawing. Application March 25, 1953, Serial No. 344,680

7 Claims. (Cl. 34-9) My invention relates to a new drying process and more particularly it relates to a process for drying moisturecontaining granular materials of high moisture diffusion resistance.

It is well known that granular materials of high moisture diffusion resistance such as, for example, ammonium nitrate, ammonium sulfate, etc. are extremely difiicult to dry to a moisture content below about 3 to 5%. Generally these materials are produced in aqueous solutions or in the presence of considerable amounts of water vapor as, for example, in the presence of steam and usually the prodduets contain considerably more than about 3 to 5% moisture, however, the amount of moisture above this range present in the materials can usually be removed with little difficulty. Reduction of the moisture content of materials of high moisture diffusion resistance below about 3 to 5% is known to require considerable drying at elevated temperatures for extended periods of time and then oftentimes only partial drying is effected. Frequently products with moisture contents as high as 1.0% are only obtained after arduous and continued drying for long periods making this phase of the process not only costly and inconvenient but only partly successful since the product still contains considerable moisture.

It is well known, of course, that many materials of high moisture diffusion resistance have a distinct tendency to cake if the moisture content is too high and this tendency is a serious disadvantage in connection with the use of such materials. Many attempts have been made to prevent or reduce caking. Such attempts have included coating the product, mixing the product with a desiccating agent, preparing the product in a form such that contact between particles is materially reduced, etc. Such measures are only satisfactory if the residual moisture of the product is reduced below a certain level. This is often not accomplished or attained only with considerable difiiculty.

It is an object of my invention to provide a method for drying materials of high moisture diffusion resistance to obtain the lowest possible moisture content.

It is a further object of my invention to provide a process for reducing the moisture content of granular materials of high moisture diffusion resistance easily, conveniently, and economically.

My invention consists essentially of mixing the moisturecontaining granular material of high moisture diffusion resistance which is to be dried with a water-insoluble, inorganic material having a high surface area and then drying the mixture in a stream of air. By employing my new invention I am able to obtain granular materials of high moisture diffusion resistance with moisture content less than 1.0% quickly and conveniently.

i It has been known previously and prior art patents such as U. S. Patent 2,399,987 have described processes whereby granular materials of high moisture diffusion resistance, such as ammonium nitrate, are coated with water-insoluble, porous inorganic materials having high surface areas to prevent the absorption of moisture and to eliminate the undesirable tendency of such materials to cake. I have ice.

found however that the addition of the water-insoluble porous inorganic material having a high surface area to the granular material of high moisture diffusion resistance, which contains undesirable amounts of moisture, prior to the drying of such latter materials greatly facilitates the drying operation both as to the eventual moisture content of the dried material which can be obtained as well as the time required to reduce the moisture content to the lowest possible level.

The water-insoluble, inorganic materials having a high surface area which I can employ in my invention include diatomaceous earth, talc, bentonite, calcium carbonate, synthetic inorganic silicates, magnesium carbonate, kaolinite, ground feldspar, fullers earth, aluminum hydroxide, dolomite, etc. I prefer to employ porous, water-insoluble, inorganic materials having a high surface area. The granular materials of high moisture diffusion resistance which I can dry according to the method of my invention include ammonium nitrate, ammonium sulfate, calcium nitrate, etc.

The water-insoluble, porous, inorganic material having a high surface area which I employ in my process is preferably used in a finely-divided state since under this condition the surface area of the added material is greatly increased. Generally, material capable of passing through a 200 mesh screen is employed however, I prefer to employ a material capable of passing through a 300 mesh screen or material even more finely divided. The amount of water-insoluble, porous. inorganic material having a high surface area which I mix in my process with the moisture-containing granular material of high moisture diffusion resistance to be dried ranges in amounts up to about 5% by weight of the material of high moisture diffusion resistance. Amounts in excess of the above range are not readily mixed with the material to be dried and the excess is generally wasted as a dust carried away in the flow of air employed in the drying operation. Furthermore, in the instance of fertilizer material such as ammonium nitrate, ammonium sulfate, etc. the nitrogen content is the important feature of the material and increasing amounts of the water-insoluble, porous, inorganic material having a high surface area merely serve to dilute the nitrogen content of the final product below the desir able level.

The temperature'at which the drying step in my process is effected ranges from about to about F. The above temperature of the product mixture during drying can be attained by preheating the mixture of moisturecontaining granular material of high moisture diffusion resistance and water-insoluble, porous, inorganic material having a high surface area and then employing drying air of a temperature such that the product mixture temperature will be maintained within the range 100 to 180 F. or the product mixture can be introduced to the dryer at room temperature and air heated to a temperature sufficient to raise the temperature of the product mixture to within the range 100 to 180 F. introduced. The stream of air in which the drying operation is effected need not be at a high rate of flow, the necessary flow being merely sufficient to remove air laden with moisture given up by the material to be dried. Absolutely dry air is not necessary in my process but it is, of course, the preferred drying medium as is evident from the nature of the drying operation.

Drying of the mixture of moisture-containing granular material of high moisture diffusion resistance and waterinsoluble, preferably porous, inorganic material having a high surface area can be effected in any convenient drying apparatus. I prefer to employ a dryer wherein the individual particles of the product mixture move with relation to one another so that the highest possible amount ofaurfaceean be, .exposedto. the drying medium. Such preferred typ d y r inc e ro r r s. fl h yre rotary dryers, screw conveyor dryers, rotary shelf dryers, etc. Dryers wherein the individual particles of the product mixture do notmove with relation to oneanother such as tunnel dryers, shelf dryers, pan dryers, etc. can be employed particularly when, the product mixture is spread ina thin layer over the drying surface.

Obviously many variations of my invention will occur to those skilled in the art such as, for example, subjecting analready dried material of high moisture diffusion resistance still containing appreciable amounts of moisture but coated with a water-insoluble, porous, inorganic material having a high surface area to further drying and I intend for such'variations and all equivalents to be ineluded Within the scope of my invention as defined in this, specification and the attached claims.

Thetollowing examples are offered to illustrate my invention and I vdo notintend to be limited to the specific materials, proportions, or procedures set forth.

' EXAMPLE 1 Ammonium nitrate at the rate of 92 pounds per hr. was mixed with diatomaceous earth in an amount equal to 3% by weight of the ammonium nitrate and the mixture heated to a temperature of 156 F. The ammonium nitrate had a moisture content of 1.5% by weight. The mixture of ammonium nitrate and diatomaceous earth was then passed at the above rate for ammonium nitrate to a steam-heated, Warm-air, flight type rotary dryer 12" in diameter and 8' long. Dry air at the rate of 609 lbs. per hr. was .passed through the dryer countercurrent to the product mixture, the temperature of the air being 103 F. The exit temperature of the air was 117 F. and the exit temperature of the product mixture was 108 F. The residencetime ofthe product mixture in the dryer was 20 min. and the dried product had a moisture content of 0.34%.

EXAMPLE II Ammonium nitrate at the rate of 95 lbs. per hr. having a moisture content of 1.6% and a temperature of 160 F. was'passedintoa dryer of the same description as that employed in Example I. .Dry air at the rate of 520 lbs. per hr. and. a temperature of 216 F. was passed through the :dryer. The residencetime of the ammonium nitrate in the dryer was 20 min. The ammonium nitrate .temperature coming from the dryer was 145 F. and the moisture content of .the product was 1.1%.

The results of Example 11 in which no diatomaceous earth was employed are to be compared with the results of Example I where. diatomaceous earth was mixed with the ammonium nitrate prior-to drying thereof. It can be seen that under essentiallysimilar conditions, the moisture content of ammonium nitrate can be reduced to about 0.3% when diatomaceous earth is mixed with the ammonium nitrate prior to drying while a final moisture content of over 1% is obtained when the ammonium nitrate is dried without mixing-it with any porous material of high surface area.

EXAMPLE III Two drying tests were conducted for comparative purposes in one ofwhich-tests diatomaceous earth was mixed with ammonium nitrate and in the other of which the ammonium nitrate was driedwithout-mixing it with any additive. The dryer employed was a steam-heated, warm air, rote-louvre rotary dryer having a diameter of 2' 7" and a length of 116". It can be seen fromv the results tabulated below that when 1% diatomaceous earth was employed the same amount of drying was obtained for approximately 4 times as much ammonium nitrate as when no diatomaceous earthwas employed.

Table Run 1' Run 2 Diatomaeeous earth, wt. percent 0 1 Feed rate, 1bs./hr 62 230 Product temp., I-- 180 168 Air temp. in, F 268 274 Air temp. out, F 217 Feed moisture, wt. percent.-. 1.0 1. 0 Product moisture, wt. percent 0.7 0.7

EXAMPLE IV A 10 gm. portion of ammonium nitrate was mixed with 3% by weight of precipitated calcium carbonate and the mixture passed. into a horizontal Pyrex tube 2 in diameter and 16 long, the tube being rotated and electrically heated. Air was flowed through the tube at the rate of 0.5 cu. ft. per min. per lb. of ammonium nitrate. The temperature of the mixture in the tube was 180 F. and the temperature of the air entering the tube was 120 F. The moisture content of the ammonium nitrate entering the tube was 2.2% by weight and the residence time of the ammonium nitrate in the tube was 15 min. The moisture content of the ammonium nitrate,

after drying was 0.81%.

EXAMPLE V Ammonium sulfate having a moisture content of 2.3%

by weight was mixed with 3% diatomaceous earth and the mixture dried in the same manner as in the previous example. The moisture content of the ammonium sulfate product after drying was 0.11%.

EXAMPLE VI Calcium nitrate having a moisture content of 2.5% by weight was mixed with 3% diatomaceous earth and the mixture dried for thirty minutes. The moisture content of the product was 0.94% which is to be compared with a moisture content of 2.45% obtained when calcium nitrate alone having an original moisture content of 3.3% was dried under the sameconditions. I

Now having described my invention, what I claim is:

1. A process for drying ammonium nitrate which comprises coating ammonium nitrate with not more than about 5% of a water-insoluble, porous, inorganic material having a high surface area and drying the coated moisture-containing material at a temperature between about 100 and 180 F. in a streamof air to obtain a coated ammonium nitrate product content.

2. A process for drying ammonium nitrate which comprises coating ammonium nitrate with not more than about 5% diatomaceous earth and drying the coated moisture-containing'material at a temperature between about 100 and 180 F. in a stream of air to obtain a coated ammonium nitrate product of reduced moisture content.

3. A drying process which comprises coating a compound selectedfrom the group consisting of ammonium nitrate, calcium nitrate and ammonium sulfate with not more than about 5% of a water-insoluble, porous, inorganic material having a high surface area and drying the coated moisture-containingmaterial at a temperature between about 100 and 180 F. in a stream of air to obtain a coated product of reduced moisture content.

4. A drying process which comprises coating calcium nitrate with not more than about 5% of a water-insoluble, porous, inorganic material having a high surface area and drying the coated moisture-containing material at a temperature between about 100 and 180 Pin a stream of air to obtain a, coated calcium nitrate product of reduced moisture content.

5. A drying process which comprises coating ammonium sulfate with not more than about 5% of a waterinsoluble, porous, inorganic material having a high surof reduced moistureface area and drying the coated moisture'containing material at a temperature between about 100 and 180 F. in a stream of air to obtain It coated ammonium sulfate product of reduced moisture content.

6. A process for drying ammonium sulfate which comprises coating ammonium sulfate with not more than about diatomaceous earth and drying the coated moisture-containing material at a temperature between about 100 and 180 F. in a stream of air to obtain a coated ammonium sulfate product of reduced moisture content.

7. A drying process which comprises coating the compound selected from the group consisting of ammonium nitrate, calcium nitrate, and ammonium sulfate with not more than about 5% diatomaceous earth and drying the coated moisture-containing material at a temperature between about and F. in a stream of air to obtain a coated product of reduced moisture content.

References Cited in the file of this patent UNITED STATES PATENTS 1,247,284 Kruger Nov. 20, 1917 2,224,284 Barnett Dec. 10, 1940 2,375,898 Bruijn et a1. May 15, 1945 2,543,898 De Vaney Mar. 6, 1951 2,561,442 Lyon et al. July 24, 1951 FOREIGN PATENTS 569,710 Great Britain June 5, 1945 

1. A PROCESS FOR DRYING AMMONIUM NITRATE WHICH COMPRISES COATING AMMONIUM NITRATE WITH NOT MORE THAN ABOUT 5% OF A WATER-INSOLUBLE, POROUS, INORGANIC MATERIAL HAVING A HIGH SURFACE AREA AND DRYING THE COATED MOISTURE-CONTAINING MATERIAL AT A TEMPERATURE BETWEEN ABOUT 100* AND 180* F. IN A STREAM OF AIR TO OBTAIN A COATED AMMONIUM NITRATE PRODUCT OF REDUCED MOISTURE CONTENT. 